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Abstract:

An aluminum slug anode usable in capacitors is produced from
multiple-stacked layers of aluminum foils. The foils are stacked
(possibly after cutting them to have an area similar to the area desired
for the anode), hot-pressed, sintered, and anodized to generate the
anode. A contact in electrical communication with the foils is formed, as
by welding a contact across at least some of the foils. A capacitor
casing be formed by situating the anode within a casing which serves as a
cathode, with the anode being wrapped in a dielectric such as separator
paper.

Claims:

1. A method of producing an aluminum slug anode for a capacitor, the
method including the steps of:a. stacking etched aluminum foils into a
multi-layer stack,b. hot pressing the multi-layer stack of aluminum
foils,c. sintering the pressed multi-layer stack of aluminum foils, andd.
anodizing the multi-layer stack of joined aluminum foils.

2. The method of claim 1 further including the step of cutting the
aluminum foils to a desired area, whereby the anodized multi-layer stack
of joined aluminum foils has at least substantially the desired area.

3. The method of claim 2 wherein the cutting step occurs prior to one or
more of the pressing and sintering steps.

4. The method of claim 2 wherein the cutting step occurs after the
sintering step.

5. The method of claim 1 wherein the hot pressing step occurs at a
temperature between 350.degree. C. and 500.degree. C.

6. The method of claim 5 wherein the hot pressing step is performed for 1
to 10 minutes.

7. The method of claim 5 wherein the hot pressing step is performed for 2
to 3 minutes.

8. The method of claim 1 wherein the hot pressing step occurs at a
temperature between 390.degree. C. and 410.degree. C.

9. The method of claim 1 wherein the hot pressing step occurs at a
pressure between 20 MPa and 40 Mpa.

10. The method of claim 1 wherein the hot pressing step occurs at a
pressure between 25 MPa and 30 MPa.

11. The method of claim 1 wherein the sintering step occurs at a
temperature between 575.degree. C. and 650.degree. C.

12. The method of claim 11 wherein the sintering step is performed between
30 minutes and 2 hours, preferably between 40 minutes and 1 hour.

13. The method of claim 11 wherein the sintering step is performed between
40 minutes and 1 hour.

14. The method of claim 1 wherein the sintering step occurs at a
temperature between 620.degree. C. and 635.degree. C.

15. The method of claim 1 wherein the sintering step is performed in a
reducing atmosphere.

16. The method of claim 1 wherein the sintering step is performed in a
primarily hydrogen atmosphere.

17. The method of claim 1 wherein at least one of the hot pressing step
and the sintering step is performed in an atmosphere primarily formed
of:a. hydrogen, orb. a forming gas.

18. The method of claim 1 wherein the hot pressing step is performed in a
reducing atmosphere.

19. The method of claim 1 wherein the step of anodizing the multi-layer
stack of joined aluminum foils includes exposing the foils to a solution
of boric acid and ammonium water to form an aluminum oxide layer.

20. The method of claim 1 wherein the anodized multi-layer stack of joined
aluminum foils has an effective surface area between 4000 cm2/cc to 9000
cm2/cc.

21. The method of claim 1 wherein the anodized multi-layer stack of joined
aluminum foils has an effective surface area between 4400 cm2/cc to 8900
cm2/cc.

22. The method of claim 1 further including the step of forming a contact
in electrical communication with the aluminum foils within the
multi-layer stack.

23. The method of claim 22 wherein the contact is formed by welding
together aluminum foils within the multi-layer stack.

24. The method of claim 1 further including the step of electrochemically
etching aluminum foils, thereby forming the etched aluminum foils.

25. The method of claim 1 further including the steps of:a. situating the
anodized multi-layer stack of joined aluminum foils within the interior
of a casing, with a contact in electrical communication with the joined
aluminum foils being exposed from the exterior of the casing;b. filling
the interior of the casing with electrolyte, andc. sealing the casing
about the joined aluminum foils and electrolyte.

26. The method of claim 25 wherein the anodized multi-layer stack of
joined aluminum foils is at least partially surrounded by separator paper
within the casing.

27. The method of claim 25 wherein the anodized multi-layer stack of
joined aluminum foils is at least partially surrounded by foil within the
casing.

28. A capacitor including one or more anodized multi-layer stacks of
joined aluminum foils formed by the method of claim 1.

29. An implantable medical device including one or more of the capacitors
of claim 28.

30. The implantable medical device of claim 29 wherein the implantable
medical device includes one or more of:a. a cardioverter, andb. a
defibrillator.

Description:

FIELD OF INVENTION

[0001]The invention relates to a method of producing an aluminum slug
anode for use in capacitors, in particular in high voltage capacitors.

BACKGROUND OF THE INVENTION

[0002]Anodes for capacitors, especially capacitors utilizing aluminum as
valve metal, are well known in the art. Bourgault et al. describe in U.S.
Pat. No. 3,345,545 a capacitor with a solid porous anode. Evans et al.
describe in U.S. Pat. No. 6,721,170 a basic capacitor design with a bulky
anode that casing be made from either tantalum or aluminum. In U.S. Pat.
No. 7,342,744 Hossick-Schott et al. describe a slug anode which casing be
made from aluminum or tantalum. Also disclosed is the use of a
cross-sectional density gradient.

[0004]Although the prior art describes many possibilities for producing
anodes for capacitors, there is still a need for anodes for capacitors
with a high energy density for special applications such as, but not
limited to, high voltage capacitors. Beside the well known applications
like flashbulbs, these capacitors are of increasing interest due to its
applications (or possible applications) in the automotive sector, in
scientific equipment and in medical devices, like defibrillators or
cardioverters-defibrillators. In implantable devices it is particularly
useful to obtain a high energy density, which allows smaller or more
efficient devices. Slug anodes are nowadays made from sintered powder,
with tantalum powder being particularly dominant. The sintered porous
materials used for anodes, mainly valve metals like tantalum or aluminum,
exhibit a large effective surface area, which is necessary for the high
energy density. Therefore tantalum and aluminum are prominent materials
used for capacitors in implantable medical devices. Beside the slug anode
configuration there are other configurations like stacked electrodes or
rolled electrodes, which both lack either high energy density or the
possibility for a compact design. The standard method for producing slug
anodes from powder material is difficult to handle for aluminium, and
thus has not been of use up to now. Therefore a method to overcome that
problem, and to produce aluminum slug anodes for high energy density
capacitors, is needed.

SUMMARY OF THE INVENTION

[0005]The present invention relates to aluminium slug anodes and
capacitors with aluminium slug anodes, as well as production methods for
producing aluminum slug anodes and capacitors with aluminum slug anodes.

[0006]One aspect of the invention is a method for producing an aluminum
slug anode wherein etched aluminum foils are stacked in a multi-layer
stack, and then hot pressed to the final is thickness of the desired
aluminum slug anode. Following the hot pressing, the hot pressed
multi-layer stack of aluminum foils is sintered, preferably (but not
necessarily) in a reducing atmosphere. This production process utilizing
hot pressed aluminum foils is easier to handle than the process with
pressed aluminum powder. If the sintered and hot pressed multi-layer
stacks do not have the desired shape, they can be cut after sintering,
but also at earlier or later stages of the process. In a next step, the
slug anode is provided with means for electrically contacting the slug
anode. In another step, the aluminum slug anode is anodized. The
foregoing process allows a simplified production of aluminum slug anodes
for use in capacitors with a high energy density.

[0007]If in the following description a "sintering" or a "sinter" process
is mentioned, it should be understood as referring to the heating of a
sample, especially (but not restricted to) heating a sample in a reducing
atmosphere to a desired temperature; holding it at that temperature for a
predefined time or duration; cooling of the sample; and removal of the
sample from the sinter furnace or other heat source.

[0008]The term "anodizing" should be understood as referring to the
process of forming a dielectric oxide at the surface of the multi-layer,
hot pressed, and sintered stack of aluminum foils.

[0009]"Electrically contacting" as mentioned above does not solely mean
establishing an electrical contact via that the parts are in direct
contact, but can also mean that the parts are joined securely with a low
electric resistance, as it casing be achieved for example (but not
restricted to) by welding, riveting, screwing, clamping, soldering or
brazing.

[0010]The etched aluminum foils can be made from high purity aluminum
foils as described below, or casing be provided as high voltage capacitor
aluminum foils. The processes for etching aluminum foils such as, but not
restricted to, electrochemical etching are well known by a person skilled
in the art. High purity aluminum foil anodes can be etched in a chloride
solution with DC, AC, or an alternation of DC and AC, or a concurrent DC
and AC current. A fine surface etching as often used for low voltage
foils is achieved mainly by AC electrolysis. A tunnel etching process as
accomplished by DC electrolysis leads to foils usable for middle and high
voltage applications. Further information can be found for example in
Technical Notes CAT.8101 D "General description of Aluminium electrolytic
capacitor", Nichicon Corporation (Japan). In a preferred version the
method of producing an aluminum slug anode for a capacitor includes
etched aluminum foils having an effective surface area between 4427
cm2/cc to 8854 cm2/cc, wherein the effective surface area is
the measured surface area per aluminum slug overall volume. Even more
preferred is an effective surface area between 5534 cm2/cc and 6640
cm2/cc. The surface area is measured using a BET surface analysing
instrument (BET signifying Brunauer, Emmett and Teller, who developed the
underlying theory for a method of determining the effective surface
area).

[0011]In a preferred version one or more of the following parameters are
used for the hot pressing process. The temperature during the hot
pressing process is preferably between 350° C. and 500° C.,
most preferably between 390° C. and 410° C. The pressure
for the hot pressing process of the multi-layer stack preferably ranges
between 20 MPa and 40 MPa, most preferably between 25 MPa and 30 MPa. The
time duration of pressing, starting with reaching and stabilization of a
desired target temperature, preferably ranges between 1 minute and 10
minutes, most preferably between 2 minutes and 3 minutes. The hot
pressing is preferably carried out in a reducing atmosphere. These
parameters allow a hot pressing process which does not compromise the
integrity of the porous foils.

[0012]In a further preferred version one or more of the following
parameters are used for the sintering process. The temperature during the
sintering process preferably ranges between 575° C. and
650° C., most preferably between 620° C. and 635° C.
The time duration of the sintering process measured within a temperature
between 575° C. and 650° C. is preferably between 30
minutes and 2 hours, most preferably between 40 minutes and 1 hour. The
reducing atmosphere is preferably realized by a H2 protected furnace.

[0013]In a further preferred version the reducing atmosphere during the
hot pressing process and/or the sintering process is a forming gas such
as (but not restricted to) 2NH3. An exemplary, (but not restricting)
reaction is 2NH3->3H2+N2.

[0014]A welding process is preferred for establishing the electrical
contact between the anode electrode and each capacitor slug anode,
especially spot welding or gas tungsten arc welding. Spot welding or
resistance welding as well as gas tungsten arc welding or tungsten inert
gas welding are well known in the art, and will not be discussed in
detail in this document.

[0015]It should also be understood that combinations of the foregoing
parameters can be useful.

[0016]Also in a preferred version the anodization of the aluminum slug
anode includes forming of an aluminum oxide layer (dielectric) in a boric
acid-ammonium water type solution. The thickness of the oxide layer is
nearly proportional to the forming voltage, with a proportional factor of
about 0.0013 micrometer/V to 0.0015 micrometer/V.

[0017]In a preferred version the method of producing an anodized aluminium
slug anode for a capacitor includes anodization of the aluminium slug
anode having an energy density capability of between 4 joules/cc of slug
to 8 joules/cc. Even more preferred is an energy density capability of
between 5 joules/cc of slug to 6 joules/cc.

[0018]In a preferred version the method of producing an aluminum slug
anode for a capacitor uses etched aluminum foils having an effective
surface area between 4000 cm2/cc to 9000 cm2/cc, preferably
between 4400 cm2/cc to 8900 cm2/cc. Even more preferred is an
effective surface area between 5500 cm2/cc and 6700 cm2/cc. The
effective surface area is measured using a BET surface analysing
instrument, as discussed above.

[0019]Since a homogenously distributed effective surface area is preferred
for the manufacturing process, it casing be favourable to use different
degrees of porosity for different layers of the multi-layer stack of
aluminum foils, or even porosity gradients within a single aluminum foil
and/or within the different stack layers. These gradients of porosity
and/or effective surface area can optimize the wetting of the slug anode
with the electrolyte, and/or improving the electrical conducting
efficiency between cathode electrode, electrolyte, and Aluminium oxide
dielectric. Additionally, the porosity gradients and/or effective surface
area gradients are adapted to improve the conducting and/or welding
properties for electrically contacting the slug anode.

[0020]Another aspect of the invention is a slug anode formed from etched
aluminum foils, which are stacked to a multi-layer stack, hot pressed,
sintered and finally anodized.

[0021]A further aspect of the invention is a method of producing a
capacitor with an aluminum slug anode wherein the anode is placed in a
separator paper into a casing with a cathode foil or in a casing with the
casing being the cathode itself. The anode and cathode are connected with
electrical feedthroughs. The anode, the separator paper and the cathode
are impregnated with an electrolyte, the interior of the case is filled
with the electrolyte, and the case is sealed. Sealing of the case can be
accomplished by several methods known in the art, such as a welding
process, preferably a laser welding process.

[0022]The invention also involves a capacitor which includes one or more
aluminum slug anodes formed from etched aluminum foils, which are stacked
to a multi-layer stack, hot pressed, sintered, and finally anodized.

[0023]Another aspect of the invention is an implantable medical device
including one or more capacitors, which include aluminum slug anodes
formed from etched aluminum foils, which are stacked to a multi-layer
stack, hot pressed, sintered, and finally anodized.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]Some aspects of the invention are schematically illustrated in the
accompanying drawings, wherein

[0025]FIG. 1a depicts a rolled etched aluminum foil from which a sheet is
formed,

[0030]FIG. 4a depicts assembly of a capacitor with an aluminum slug anode,
and

[0031]FIG. 4b depicts an assembly of a capacitor with multiple slug
anodes.

DETAILED DESCRIPTION OF THE INVENTION

[0032]A preferred method for producing aluminum slug anodes starts with
the use of etched aluminum foils, which can be purchased ready-made, or
etched by known processes like electrochemical etching with a chloride
solution. The etched aluminum foil 10 shown in FIG. 1a is then cut into
handy pieces 11 of roughly or precisely the size of the later aluminum
slug anode, as shown schematically in FIG. 1a. These pieces are then
assembled to a multi-layer stack 12, like in FIG. 1b. The multi-layer
aluminum stack is then hot pressed at a temperature between 350°
C. and 500° C., preferably between 390° C. and 410°
C., and with a pressure applied between 20 MPa and 40 MPa, preferably 25
MPA to 30 MPa, for a duration of between 1 minute to 10 minutes,
preferably between 2 minutes and 3 minutes. The application of pressure
should be started when the desired target temperature has been reached
and stabilized. The complete hot processing is done in a reducing
atmosphere. In FIG. 2a a multi-layer aluminum stack 12 is shown in a hot
pressing setup with the pressing tool 22 and a heating 23. FIG. 2b shows
a schematic view of the hot pressed multi-layer aluminum stack, already
having the final size of the later slug anode. The hot pressing procedure
is followed by a sintering process in a reducing atmosphere like in a H2
furnace. The temperature for the sintering process is between 575°
C. and 650° C., preferably between 620° C. and 635°
C. The time duration of the sintering process measured at a temperature
between 575° C. and 650° C. is between 30 minutes and 2
hours, preferably between 40 minutes and 1 hour. After the sintering
process, the pressed and sintered aluminum slug anode is cut to the
desired precise form, if not done so earlier. Furthermore the electrical
contact between the anode electrode and each capacitor slug anode has to
be established, preferably by a welding process like spot welding or gas
tungsten arc welding. An example is shown in FIG. 3, which illustrates
joining an electrically conducting attachment 31 to an aluminum slug
anode 30 by a spot welding process utilizing a welding electrode 32 and
welding tongs 33. In a last step the aluminum slug anode is anodized,
preferably in a boric acid-ammonium water type solution and under
application of a voltage proportional to the desired oxide thickness.

[0033]The assembly of a capacitor 40a and 40b utilizing the anodized slug
anode is shown in FIGS. 4a and 4b. While FIG. 4a illustrates the assembly
using a single anodized aluminum slug anode 43a, FIG. 4b illustrates the
assembly using anodized multiple aluminum slug anode 43b. For the
capacitor assembly, the anode slug 43a is wrapped in separator paper 42
and then placed in a casing 41', preferably an aluminum casing. This
casing acts as the cathode of the capacitor, and includes a contact 47
and a hole 46, which forms a feedthrough in combination with the contact
of the anode 44a (and an insulator 45a) after sealing the capacitor. The
capacitor 40b in FIG. 4b is assembled similarly to the assembly of the
capacitor 40a in FIG. 4a. The anodized multiple slug anode 43b is wrapped
in separator paper 42 and then placed in a casing 41', preferably an
aluminum casing. This casing acts as the cathode of the capacitor, and
includes a contact 47 and a hole 46 which forms a feedthrough in
combination with the contact of the anode 44b and an insulator 45b after
sealing the capacitor. The slug anodes of the multiple slug anode can be
separated by an insulator, like paper, and/or a cathode foil, preferably
an aluminum cathode foil, or combinations of these (which can lead to a
layer of insulator, cathode foil, insulator, etc.).

[0034]The anode slug or the multiple anode slug can be wrapped into the
separator paper in different ways, such as having the anode slug or
multiple anode slug enclosed completely by the separator paper, or having
the separator paper covering only the flat upper 49a and 49b surface and
bottom surface but not the lateral area 48a and 48b.

[0035]The sealed capacitor can be used as known in the art in implantable
devices, especially in defibrillators or cardioverters-defibrillators.

[0036]A person skilled in the art will understand that any combinations of
the previous components/structures and steps may be used, and the listed
versions shall not limit the scope of the invention. It will also be
apparent to those skilled in the art that numerous modifications and
variations of the described examples and versions are possible in light
of the foregoing discussion. The disclosed examples and versions are
presented for purposes of illustration only. Therefore, it is the intent
to cover all such modifications and alternate versions as may come within
the true of the claims below.